This invention relates to a semiconductor device, more particularly an integrated circuit device of a high breakdown voltage.
Generally a semiconductor device, in particular a bipolar integrated circuit device comprises, as shown in FIG. 1, a semiconductor substrate 1 of one conductivity type and two or more impurity regions 2 and 3 of the opposite conductivity type formed in the substrate 1 and arranged at predetermined intervals. The regions 2 and 3 have surfaces flush with that of the substrate 1 and constitute emitter E and collector C of, for example, a lateral transistor, respectively. On the surface of the substrate 1 including the surfaces of the regions 2 and 3, an insulative film 4 is locally formed. Further, on the film 4 and the surfaces of the regions 2 and 3, aluminum electrodes 5 are provided.
The substrate 1 is made of silicon. The film 4, which protects the lateral transistor, is usually made of silicon dioxide (SiO.sub.2) which is stable both physically and chemically. The film 4 should be made of clean SiO.sub.2 containing no contaminants. Otherwise, the contaminants would accumulate in the film-to-substrate interface 1A to build up a leak current. A leak current would lower the breakdown voltage of the PN junction, thereby reducing the reliability of the lateral transistor.
A leak current will flow even if the film 4 is made of clean SiO.sub.2. More specifically, a current can flow in the emitter E and collector C of the lateral transistor, even if no input voltage is applied on the base B. This means a short circuit has occured between the emitter E and collector C, as often takes place in a discrete transistor. The lateral transistor must be replaced by a new one if a leak current flows in it.
The inventors of this invention consider that the MOS structure constituted by the silicon substrate 1, insulative film 4 and aluminum electrodes 5 is responsible for such a leak current. That is, despite the threshold voltage specific to the MOS lateral transistor, positive charge accumulates between the substrate 1 and the electrodes 5 to form an inversion channel 6 when a voltage is applied on the electrodes 5. Through this channel 6 the emitter E and collector C are shortcircuited, and a leak current therefore flows between them. It is ascertained that the leak current affects the current amplification factor .beta. of the transistor, which is expressed as .beta.=I.sub.C /I.sub.B, where I.sub.C and I.sub.B denote collector current and base current, respectively. The leak current renders the base current I.sub.B unstable, and the current amplification factor .beta. becomes unstable.
To avoid a leak current it is sufficient to thicken the SiO.sub.2 film 4 to raise the threshold voltage of the MOS lateral transistor and thereby prevent formation of an inversion channel 6. If the SiO.sub.2 film 4 is made thicker, however, the aluminum electrodes 5 will more likely be broken when wires are led out from them.
In known integrated circuit devices a diffusion layer of a high impurity concentration, which is called "channel-diffusion layer" or "channel stopping zone", is formed to surround semiconductor elements such as transistors so as to avoid the channel effect. If an electrode metal layer is formed on the diffusion layer, an electric field intensity at N.sup.+ N or P.sup.+ P junction surfaces is more intense than at any other portions. As a result, the widthstand voltage at the N.sup.+ N or P.sup.+ P junction surfaces will become too low.